In general, a membrane filtration apparatus of an immersion type using a hollow fiber membrane is used in such manner that the membrane filtration apparatus is immersed in a septic tank, and suction-filters sewage containing suspended substances through a hollow fiber membrane, thereby providing filtered water.
The hollow fiber membrane is often used by bundling a large number of hollow fiber membranes in thread form to provide a cylindrical cartridge. In the cartridge, a large number of the hollow fiber membranes are bundled, one end of which is closed, and the other end of which is opened. The sewage is sucked from the open ends of the hollow fiber membranes to provide filtered water through the hollow fiber membranes.
The water collecting efficiency of the membrane filtration apparatus is proportional to the suction power. For increasing the water collecting amount, it is effective to immerse a large number of the cartridges in the septic tank. This is because a large water collecting amount can be obtained by using a large number of the cartridges simultaneously even though the water collecting amounts of the individual cartridge are small.
Patent Document 1 discloses a filtration apparatus, in which four filter submodules each containing hollow fiber membranes in the form of a cylindrical cartridge are fastened to provide a unit, and a plurality of units are stacked and fixed to a frame. In the filtration apparatus, the upper parts of the four filter submodules are fixed liquid-tightly to one manifold. A water collecting tube is engaged with the manifold, and filtered water from four filter submodules can be collected from one manifold.
A large number of the units containing four filter submodules are aligned to fabricate one membrane filtration apparatus.
There is a problem that the suspended substances in the sewage are attached to the hollow fiber membrane to decrease the filtration efficiency. Accordingly, a diffuser is provided at the lower part of the filter submodules, and air is applied to the modules periodically. The suspended substances attached to the hollow fiber membranes are detached and removed with the air. The process is referred to as air scrubbing.
Patent Document 1: JP-T-2002-542013